The invention relates to electrical switches, and specifically to electrical switches which are used in an environment containing explosive vapors or gases.
The use of electrical equipment in areas where explosive vapors or gases are present presents a potentially hazardous situation. Since many electrical components can cause arcing, there is the distinct possibility that the explosive vapors or gases could be ignited by an arc, causing an explosion. For this reason, safety codes require that certain precautions be taken when any electrical system which includes an arcing device is installed in an area containing explosive vapors or gases.
Electrical switches are one type of electrical device which can generate an arc; therefore safety measures must be taken if an electrical switch is to be used in an environment containing explosive gases or vapors. Generally, three techniques are used to allow an electrical switch to be safely used in an explosive environment. One technique is to install a conventional electrical switch in an explosion-proof enclosure designed to contain the internal explosions of the gases. This requires all conduit or cable entrances to the enclosure be sealed to prevent the propagation of the explosion. Conduit or cable seals and the additional installation labor add cost to the installation.
Another technique used is to install an electrical switch that is factory sealed. This eliminates the need for conduit or cable seals, in most cases. Factory sealed switches can be either hermetically sealed or contained in an explosion-proof enclosure. Generally, hermetically sealed switches have limited electrical ratings and are expensive. Factory sealed switches contained in their own explosion-proof enclosures do not have the limited electrical rating of the hermetically sealed switch, but have the disadvantage of being relatively costly.